1. Field of the Invention
The present invention relates to a superconducting thin film and a process for producing the same. More particularly, it relates to a superconducting thin film having an improved high current density of superconductivity and a process for producing the same.
The superconducting thin film according to the present invention possesses improved surface flatness as well as a high critical temperature and a high current density and is advantageously applicable in a variety of electronics devices including the integrated circuits.
The present invention also relates to a superconducting wire and a process for producing the same. More particularly, it relates to a superconducting wire having an improved high current density of superconductivity and a process for producing the same.
The superconducting wire according to the present invention is advantageously applicable in the field of electric power transportation and in a variety of wiring materials for electronics devices.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a kind of phase changes of electrons under which the electric resistance become zero and the perfect diamagnetism is observed.
In the field of electronics, a variety of superconducting devices are known. A typical application of the superconducting device is Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors.
The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensor or detector for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a precise quantum phenomenon. Development of the superconducting devices such as high-speed logic units or no power-loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
The superconducting wire is demanded in the field of electromagnetic coils or a variety of electric parts as well as in the field of electric power transportation. For example, in the high-speed computers, the superconducting wires is demanded as a wiring material for IC packaging or the like for delivering current without any loss of power.
However, the critical temperature "Tc" of superconductivity could not exceed 23.2K of Nb.sub.3 Ge which was the highest Tc for the past ten years.
The possibility of an existence of a new type of superconducting material having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30K which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90K in February 1987. And hence, the possibility of an existence of high-temperature superconductors have burst onto the scene.
It is said that the superconducting properties of the above-mentioned new type compound oxide superconductors are influenced by the oxygen deficiency in the crystal. In fact, if the oxygen deficiency do not exist in the crystal, Tc can not be higher and a discrepancy between the on-set temperature and a temperature where perfect zero resistance is observed become big.
Generally, the thin films composed of the above-mentioned new type superconducting materials are prepared by physical vapour deposition technique. However, the critical current density "Jc" observed in the superconducting thin film obtained is very low even if the thin film possesses a high critical temperature "Tc". Such low critical current density is a problem when the thin films are utilized as superconductors in practical uses.
The superconducting wires composed of the compound oxides are generally produced by a wire-drawing technique in which sintered powder material of the compound oxide is compacted in a metallic sheath which is then drawn. In the superconducting wires produced by the conventional technique, however, it is difficult to improve the critical current density even if the superconducting wire shows a high critical temperature Tc. In fact, since the compound oxide is an anisotropic crystal in which electric current flow mainly along its c-axis, the maximum current density of the conventional superconducting wire which is produced by the conventional technique in which powders of such compound oxide is compacted at random is limited to a value which is insufficient to use the wire in practical uses.
In the conventional sputtering technique for preparing a thin film composed of the ceramics type oxide superconductors disclosed in Japanese patent laid-open No. 56-10,9824, sputtering is effected in oxygen-containing atmosphere while the substrate is heated in order to improve ordering or orientation of the crystal and the resulting film deposited on the substrate by the sputtering stage is further heat-treated in order to increase the oxygen-content in the thin film.
This patent teaches that the superconducting thin film which is represented by the formula BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 005.ltoreq.x.ltoreq.0.35) and which is prepared in an oxygen-containing atmosphere by high-frequency sputtering technique should be further heat-treated at 500.degree. to 550.degree. C. but mentions nothing about the conditions how to prepare the thin film of high-Tc superconductor which was discovered after the filing date of this patent.
The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor in the following patent applications: U.S. patent application Ser. No. 152,714 filed on Feb. 2, 1988, U.S. patent application Ser. No. 167,895 filed on Mar. 13, 1988, U.S. patent application Ser. No. 195,145 filed on May 18, 1988, U.S. patent application Ser. No. 195,147 filed on May 18, 1988, U.S. patent application Ser. No. 200,206 filed on May 31, 1988 or the like.
Although the processes disclosed in these patent applications are themselves useful and satisfactory, it is still requested to improve further the critical temperature and particularly the critical current density. The present invention is an extension of and an improvement in these patent applications.
Therefore, an object of the present invention is to provide an improved thin film which is composed of compound oxide type superconducting material and which has a high critical density of superconductivity in addition to the high critical temperature and a process for producing the same.
Another object of the present invention is to solve the above-mentioned problem and to provide an improved compound oxide type superconducting wire having higher current density.